Effets de régimes hyperlipidique et cafeteria sur le développement de l'obésité et ses désordres associés chez la souris

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Effets de régimes hyperlipidique et cafeteria sur le

développement de l’obésité et ses désordres associés chez

la souris

Charles Desmarchelier

To cite this version:

Charles Desmarchelier. Effets de régimes hyperlipidique et cafeteria sur le développement de l’obésité et ses désordres associés chez la souris. Alimentation et Nutrition. AgroParisTech, 2010. Français. �NNT : 2010AGPT0022�. �pastel-00601961�

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N°: 2009 ENAM XXXX

AgroParisTech

Molecular Nutrition Unit, Technische Universität München

présentée et soutenue publiquement par

Charles DESMARCHELIER

Le 8 mars 2010

Effects of high fat and cafeteria diets on obesity development and

associated metabolic disturbances in mice

Doctorat ParisTech

T H È S E

pour obtenir le grade de docteur délivré par

L’Institut des Sciences et Industries

du Vivant et de l’Environnement

(AgroParisTech)

Directeur de thèse : Hannelore Daniel

Jury

M. Daniel TOME, Professeur, Agroparistech Examinateur

M. Dominique BAUCHART, Directeur de Recherche, INRA Rapporteur

M. Xavier BIGARD, Professeur, CRSSA Rapporteur

M. Mihai COVASA, Professeur, Pennsylvania State University Examinateur

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Acknowledgments

I would like to thank my supervisor Prof. Hannelore Daniel for giving me the chance

to do this project and for her support and guidance over the years.

I would like to thank the members from the Nusisco program and committee for

funding this project and for all the meetings and events.

I would like to thank Daniel Tome and Gilles Fromentin for their help for my

inscription in this PhD program and for the organization of my PhD defense.

I would like to thank my examiners and my reporters for accepting to examine my

thesis.

I would like to thank everyone at ZIEL, especially Dr. Thomas Clavel, Adelmar

Stamfort, Tobias Ludwig, Christoph Dahlhoff and Tanja Heidler for the work done in

common and all the discussions.

I would like to thank Prof. Jimmy Bell, Jelena Anastasovska, Mohammed Hankir,

Michael Russ and everyone at the BIC for having hosted me 6 months in their lab in

London.

I would like to thank my mother and my sister for their constant support over these

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Finally, I would like not to thank all the people who kept enticing me into climbing and

were often successful at it during these last 3 years, especially Tobias, Korbi,

Thomas, Hase, Tilo, Alex, Frosch, Claus, Dave, Simon, Michael, Trev, Andy and all

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Abstract

Introduction: Obesity results from a prolonged imbalance between energy intake and energy expenditure, as depending on basal metabolic rate, heat production,

thermogenic effects of the diet and physical activity. Diet-induced obesity (DIO) in

rodents can be achieved by different regimens and approaches. Diets providing a high fat intake have been established as a “gold standard” to generate obese rodent

models and have proven to initiate pathologies similar to those encountered in

humans. However, this dietary treatment is far from being standardized and its

relevance has been criticised on the basis of findings in humans that total energy

intake rather than fat per se determines body fat accumulation in humans. Hence,

cafeteria diets have been introduced by providing a choice of several palatable food

items of variable composition, appearance and texture in addition to a non-purified

diet. Those approaches have been shown to induce obesity by a hyperphagia.

Objective: This thesis aimed at comparing the effects of a high fat vs. a cafeteria diet on food intake, weight gain and determinants of energy homeostasis and metabolism

in obese mice. Results: Our key findings demonstrate that both a high fat and a

cafeteria diet were almost equally efficient in driving an obese phenotype but did not

necessarily elicit the same metabolic changes. The cafeteria diet as characterised by

a higher carbohydrate (mainly sucrose) and lower fat content seemed to be more

deleterious for liver steatohepatosis and provoked more pronounced changes in the

gut microbiota. Despite a lower cholesterol content than in the high fat diet, mice fed

the cafeteria diet presented levels of circulating cholesterol as high as animals on a

high fat diet. Changes in gene expression in liver and intestine suggested an

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pronounced effects of the two high calorie diets causing obesity when compared to

animals on control diet remaining lean vanished when diets with identical

composition were supplied in powder form and not as standard pellets. Here, even

the control diet with a high starch but very low fat content caused a substantial weight

gain with only minor differences to the two other high-calorie diets. Conclusion: The

results presented here raise the question of whether high fat diets used for induction

of obesity are the proper models to simulate human obesity and its pathologies.

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Résumé

Introduction : L’obésité est causée par un déséquilibre prolongé entre les apports

énergétiques et l’activité physique, dépendant du métabolisme de base, de la

production de chaleur et des effets thermogéniques du régime et de l’activité

physique. Chez les rongeurs, l’obésité induite par le régime peut être obtenue par

différents régimes et approches. A cet égard, les régimes hyperlipidiques sont considérés comme les régimes de référence pour générer des modèles de l’obésité

chez le rongeur et engendrent des pathologies similaires à celles rencontrées chez l’homme. Cependant, ce régime alimentaire est loin d’être standardisé et a été

critiqué sur le fait que la prise énergétique totale et non uniquement les lipides

régissait l’accumulation de graisse corporelle chez l’homme. Ainsi, les régimes

cafétéria ont été introduits : en offrant en plus d’un régime non purifié un choix de

plusieurs aliments appétants, de composition, d’apparence et de texture différentes, ils permettent le développement de l’obésité en déclenchant l’hyperphagie. Objectif :

L’objet de ces travaux a été de comparer chez des souris obèses les effets d’un

régime hyperlipidique à ceux d’un régime cafétéria sur la prise de nourriture, la prise

de poids et les déterminants du métabolisme et de l’homéostase énergétique.

Résultats : Nos résultats démontrent qu’un régime hyperlipidique et un régime

cafeteria permettent tous deux d’obtenir un phénotype obèse mais sans causer

nécessairement les mêmes changements métaboliques. Le régime cafétéria,

caractérisé par un contenu en glucides (principalement le sucrose) plus élevé et un

contenu en lipides plus faible, semble avoir des conséquences plus néfastes pour le

foie et provoque des changements plus prononcés au niveau du microbiote

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cholestérolémie similaire. Les niveaux d’expression des gènes impliqués dans le

métabolisme du cholestérol dans l’intestin grêle et le foie suggèrent une

augmentation de la synthèse de cholestérol de novo et une modification de son

transport, ces effets étant plus marqués chez les souris nourries au régime

hyperlipidique. Conclusion : Ces résultats remettent en question le statut des régimes hyperlipidiques pour déclencher l’obésité et pour générer ses pathologies

associées. Les régimes cafétéria sont aussi efficaces à cet égard et sont plus proches des régimes consommés chez l’homme.

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Keywords

High fat diet, cafeteria diet, food texture, food intake, adipose tissue, small intestine,

intrahepatic triacylglyceride, cholesterol metabolism, gut microbiota, microarrays,

Fourier transform infrared spectroscopy, obesity

Mots clés

Régime hyperlipidique, régime cafeteria, texture alimentaire, prise alimentaire, tissu

adipeux, intestin grêle, triacylglyceride intrahépatique, métabolisme du cholestérol,

microbiote intestinal, puce à ADN, Spectroscopie infrarouge à transformée de

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Declaration of contributors

Experiments of Study 1 of this thesis were conducted at:

Molecular Nutrition Unit

ZIEL - Research Center for Nutrition and Food Sciences

Technische Universität München (TUM)

Gregor-Mendel-Str. 2

85350 Freising - Weihenstephan

Germany

Clinical Nutritional Medicine

Germany

Molecular Nutritional Medicine

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Germany

Friedrich Schiller University

Institute of Nutrition

Dornburger Str. 24

07743 Jena

Germany

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Biofunctionality

Germany

Microbiology

Weihenstephaner Berg 1

85350 Freising – Weihenstephan

Germany

Lehrstuhl für Biologische Chemie

Emil-Erlenmeyer-Forum 5

85350 Freising-Weihenstephan

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List of figures

Part 1: Literature review

Figure 1.1. Body fat mass as a function of body weight in 43 strains of inbred mouse

fed a high fat diet for 8 weeks.

Figure 1.2. Summary of the effects of high fat diets on the adipose tissue.

Figure 1.3. Summary of the effects of high fat diets on the liver.

Figure 1.4. Summary of the effects of high fat diets on the small intestine.

Figure 1.5. Summary of the effects of gut microbiota on host metabolic and

inflammatory processes.

Part 2: Experimental work conducted Study 1

Figure 2.1. Body weight developments in animals receiving the different diets during

12 or 18 weeks of feeding.

Figure 2.2. Liver weight and IHTG in animals receiving the different diets.

Study 2

Figure 3.1. Body weight developments in animals fed the different diets.

Figure 3.2. Glucose tolerance in animals fed for 9 weeks the different diets.

Figure 3.3. Selected blood parameters.

Figure 3.4. Cholesterol balance data obtained from animals fed the different diets for

12 weeks.

Figure 3.5. Cholesterol and TG contents in intestine and liver of animals fed the

different diets for 12 weeks.

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Figure 3.7. The effect of dietary fat on the expression of genes related to cholesterol

and lipid metabolism in the small intestine.

Study 3

Figure 4.1 Cumulative body weight gain over the 12 week feeding trial.

Figure 4.2 Spectral analysis revealed distinct chemical patterns.

Supplemental figure 4.1 Calculation of bacterial proportions using FISH coupled with

flow cytometry.

Supplemental figure 4.2 Dot plots showing fluorescence signals obtained with the

16S rRNA probe Ecyl-0387 in both cohorts of mice.

Supplemental figure 4.3 Dot plots showing the purity of sorted bacteria hybridized

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List of tables

Part 1: Literature review

Table 1.1. Single gene mutations associated with an obesity phenotype

Study 1

Table 2.1. Composition of the different diets employed.

Table 2.2. Final body weight, cumulative food, energy, water and macronutrient

intake in animals receiving the different diets either provided in pellet or powder form.

Table 2.3. Serum clinical chemistry and adipokine levels.

Table 2.4. Organ weight.

Table 2.5. Relative expression of selected target genes in visceral adipose tissues.

Study 2

Table 3.1. Diet composition.

Table 3.2. Primer sequences.

Table 3.3. Effect of a chronic high fat and cafeteria diet on final body weight,

cumulative food, energy, water, macronutrient and cholesterol intake and energy

assimilation according to the dietary treatment.

Table 3.4. Effect of a chronic high fat and cafeteria diet on the expression of genes

related to cholesterol and lipid metabolism in the small intestine.

Table 3.5. Effect of a chronic high fat and cafeteria diet on the expression of genes

related to cholesterol in the liver.

Study 3

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Table 4.2 Group-specific 16 rRNA oligonucleotide probes used for in situ

hybridization.

Table 4.3 Final body weight, cumulative energy, water and macronutrient intake

according to the dietary treatment.

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Abbreviations

11-β-HSD-1: 11β-hydroxysteroid dehydrogenase type 1

Abca1: ATP-binding cassette, sub-family A, 1

Abcg5 : ATP-binding cassette, sub-family G, 5

Abcg8 : ATP-binding cassette, sub-family G, 8

ACC: acetyl CoA carboxylase

Acc1: acetyl-CoA carboxylase 1 Actb: β-actin

ALT: alanine aminotransferase

AMP: adenosine monophosphate

AMPK: AMP-activated protein kinase

Apo: apoplipoprotein

ARH(1): heterogeneous autoregressive order one

AST: aspartate aminotransferase

ATGL: adipose triacylglycerols lipase

ATP: adenosine triphosphate

BAT: brown adipose tissue

BMI: body mass index

Bp: base pairs

Cal: calorie

CD14: cluster of differentiation 14

CD36: cluster of differentiation 36

ChREBP: carbohydrate response element binding protein

CLA: conjugated linoleic acid

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CoA: coenzyme A

CPT1: carnitine palmitoyl transferase-1

Cyp27a1: cytochrome P450, family 27, subfamily a, polypeptide 1

Cyp51: cytochrome P450, family 51

DHA: docosahexaenoic acid

Dhcr7: 7-dehydrocholesterol reductase

DIO: diet-induced obesity

EAT: epididymal adipose tissue

EDTA: ethylenediaminetetraacetic acid

EPA: eicosapentaenoic acid

Fas: fatty acid synthase

FATP-4: fatty acid transport protein 4

Fiaf: fasting-induced adipose factor

FISH: fluorescent in situ hybridization

FT-IR: Fourier transform infrared

GAPDH: glyceraldehyde 3-phosphate dehydrogenase

GE: gross energy

GF: germ-free

GIP: gastric inhibitory polypeptide

GLP-1: glucagon-like peptide-1 Gpr41: G protein-coupled receptor 41 Gpr43: G protein-coupled receptor 43 HDL: high-density lipoprotein Hmgcr: 3-hydroxy-3-methylglutaryl-Coenzyme A reductase Hmgcs2: 3-hydroxy-3-methylglutaryl-Coenzyme A synthase 2 Hprt: hypoxanthine phophoribosyltransferase

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Idh1: isocitrate dehydrogenase 1 (NADP+), soluble

I-FABP: intestinal fatty acid binding protein

IHTG: intrahepatic triacylglyceride

IRS-1: insulin receptor substrate 1

IRS-2: insulin receptor substrate 2

LDL: low-density lipoprotein

LDLr: LDL receptor

L-FABP: liver fatty acid binding protein

LPL: lipoprotein lipase

LPS: lipopolysaccharide LXRα: liver X receptor α

MAT: mesenteric adipose tissue

MCP-1: monocyte chemotactic protein-1

ME: metabolizable energy

Me1: malic enzyme 1, NADP(+)-dependent, cytosolic

MTP: microsomal triacylglyceride transfer protein

mRNA: messenger ribonucleic acid

MUFA: mono-unsaturated fatty acid

Mvd: mevalonate decarboxylase

N: nitrogen

NADP: nicotinamide adenine dinucleotide phosphate

NAFLD: non-alcoholic fatty liver disease

Npc1l1: Niemann-Pick C1-like protein 1

Nsdhl: NAD(P) dependent steroid dehydrogenase-like

PBS: phosphate buffered saline

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Pgc-1α: peroxisomal proliferator activated receptor coactivator 1α

Pmvk: phosphomevalonate kinase

PUFA: poly-unsaturated fatty acid

PYY: peptide YY

qPCR: real-time quantitative polymerase chain reaction

RT: room temperature

Scarb1: scavenger receptor class B, member 1

Scd1: stearoyl-Coenzyme A desaturase 1

SCFA: short-chain fatty acids

SDS: sodium dodecyl sulfate

SFA: saturated fatty acid

Slc25a1: solute carrier family 25 (mitochondrial carrier, citrate transporter), member 1

SR-B1: scavenger receptor class B1

SREBP-1: sterol response element binding protein 1

SREBP-2: sterol response element binding protein 2

TG: triacylglycerols

TLR-4: toll-like receptor 4

Tm7sf2: transmembrane 7 superfamily member 2

UCP: uncoupling protein

UN: unstructured

V/v: ratio volume to volume

VLDL: very low-density lipoprotein

W/v: ratio weight to volume

W/w: ratio weight to weight

WAT: white adipose tissue

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Publications and scientific communications

Publications:

Clavel T., Desmarchelier C., Binder U., Wenning M., Skerra A. ,Haller D., Daniel H.

Chemical and phylogenetic alterations of mouse cecal microbiota induced by diet and

obesity. Submitted to Journal of Nutrition (in revision).

Desmarchelier C., Ludwig T., Bader B.L., Klingenspor M., Daniel H. Diet-induced

obesity in ad libitum fed mice: food texture overrides the effect

of macronutrient composition. Submitted to Journal of Nutrition (in revision).

Desmarchelier C., Dahlhoff C., Keller S., Sailer M., Daniel H. A cholesterol-paradox:

a high fat diet induces major changes in intestinal

cholesterol metabolism with reduced tissue levels despite a plasma

hypercholesterolemia. Submitted to Journal of Lipid Research.

Communications:

Desmarchelier C., Dahlhoff C., Keller S., Sailer M., Daniel H. A cholesterol-paradox: a high fat diet induces major changes in intestinal

cholesterol metabolism with reduced tissue levels despite a plasma

hypercholesterolemia. Poster communication at the XI International Conference on

Obesity, July 2010, Stockholm, Sweden.

Desmarchelier C., Ludwig T., Bader B.L., Klingenspor M., Daniel H. Diet-induced obesity in ad libitum fed mice: food texture overrides the effect

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of macronutrient composition. Poster communication at the XI International

Conference on Obesity, July 2010, Stockholm, Sweden.

Desmarchelier C., Daniel H. Intestinal cholesterol metabolism in obese mice. Poster communication at the annual meeting of Société Française de Nutrition (SFN),

December 2009, Montpellier, France.

Desmarchelier C., Daniel H. Comparison of the effects of different obesigenic diets on mice. Poster presentation for scientist of Unilever R&D September 2009,

Vlaardingen, The Netherlands.

Desmarchelier C., Daniel H. The intestine as a target for obesity-associated disturbances in signalling and function. Poster presentation for scientist of Unilever

R&D September 2008, Vlaardingen, The Netherlands.

Desmarchelier C., Daniel H. The intestine as a target for obesity-associated disturbances in signalling and function. Poster presentation for scientist of Unilever

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General introduction

Obesity incidence has increased rapidly over the last two decades, reaching an

epidemic state. An incidence of above 20% has been observed in most Western

countries (WHO 2007), particularly in the US and UK, with an alarmingly high

incidence among children (Rocchini 2002). The estimated number of overweight

children globally in 2005 was at least 20 million (WHO 2005). Obesity is defined as a

body mass index (BMI) of 30 kg/m2 or more and overweight is defined as a BMI of 25 kg/m2 or more by the World Health Organization (WHO) (WHO 2005). However, BMI is not necessarily the best parameter in defining obesity, and in particular not for

predicting obesity-associated metabolic problems.

Obesity is known to increase the risk of developing a variety of disorders including

type 2 diabetes, coronary heart disease, osteoarthritis, as well as certain types of

cancer and psychological problems (Pi-Sunyer 1993; Must et al. 1999; Visscher and

Seidell 2001; Calle and Kaaks 2004). It is also highly associated with the metabolic

syndrome, which includes conditions such as impaired glucose tolerance, insulin

resistance, dyslipidemia and hypertension (Grundy 2004; Grundy et al. 2004). This

condition is a worldwide problem, and is showing a worrying increase in developing

countries as well as developed countries (Prentice 2006).

The dramatic rise in obesity and the metabolic syndrome are a consequence of

several lifestyle factors in modern societies. Factors such as nutrition, physical

activity, smoking, alcohol and stress are well known lifestyle components associated

with the development of obesity-associated diseases (Ueno et al. 1997). Modern

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nature of many jobs with technological developments and the increasing use of

computers for everyday tasks as well as readily available, high calorie, ready-made

diets.

Thus, there is currently a great need fot effective therapies. Numerous medical and

behavioural interventions have been tried to treat obese patients but only a few were

successful. Pharmacological compounds often had to be withdrawn, due to severe

undesired side effects (Farrigan and Pang 2002). Bariatric surgery is considered the

most successful treatment in highly obese patients but the significant risk of complications does not allow its wide‐range use (Sjostrom et al. 2004). Therefore, there is a huge challenge for the scientific community to search for more effective

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I.I. Diet-induced obesity in animal models

I.I.1. Animal models of obesity

To understand the genetic and environmental basis of obesity, animal models have

proven useful by allowing manipulations technically or ethically not feasible in

humans (Speakman et al. 2008). In these models, obesity can be induced by genetic

mutations (See Table 1.1 for spontaneous single gene loss-of-function mutations),

pharmacologically, by injecting gold thioglucose for example (Brecher and Waxler

1949) or by a variety of dietary manoeuvres.

Table 1.1. Single gene mutations associated with an obesity phenotype

Candidate gene Syndrome

NTRK2 neurotrophic Tyrosine Kinase Receptor Type 2

GPR24 G protein-coupled receptor 24

PCSK1 Proprotein convertase subtilisin/kexin type 1

POMC Proopiomelanocortin

LEP leptin (obesity homolog, mouse)

CRHR2 corticotropin releasing hormone receptor 2

MC4R melanocortin 4 receptor

CRHR1 corticotrophin releasing hormone receptor 1

LEPR leptin receptor

SIM1 single-minded homolog 1 (Drosophila)

MC3R melanocortin 3 receptor

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These models of obesity have allowed insights into some critical pathways but their

overall relevance is nonetheless questionable since common obesity cannot be

attributed to a single gene or single pathway. Thus, the polygenic nature of obesity

calls for a more realistic approach to generate animal models of obesity. In this

respect, diet-induced obesity allows to mimic situations more closely related to what

can be observed in humans.

I.I.2. High fat diets in diet-induced obesity

I.I.2.1. The effect of high fat diets on the development of obesity

As early as 1951, Fenton and Carr observed that, when providing diets with

increasing fat content to rodents, some strains showed marked weight gains, while

others had a much less pronounced response. They reported elevated food

utilization with diets high in fat and showed by carcass analysis that the strain

responding well to high fat feeding accumulated most of the excess weight as fat

(Fenton and Carr 1951). In 1955, Mickelsen et al. showed for the first time in rats that

obesity could be achieved by feeding a diet high in fat and implied that this could be

due to an excess consumption of calories (Mickelsen et al. 1955). High fat diets are

now fairly well accepted to model the disorders of human obesity in rodents (Buettner

et al. 2007) and have since then been extensively used to induce obesity in animal models. A PubMed search in December 2009 with the keywords “high fat diet” and

“obesity” retrieved more than 1500 results, mainly animal studies. To understand the

mechanisms behind the excess storage of energy usually associated with feeding

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I.I.2.2. High fat diets and hyperphagia

The most obvious, and possibly easiest, parameter to look at is the energy intake,

which is simply calculated by measuring the food consumption and multiplying it by

the energy density of the diet. High fat feeding has usually been associated with

hyperphagia, meaning the group given the high fat diet tended to consume more

calories than the control group. This effect has been observed in mice (Mercer and

Trayhurn 1987; West et al. 1995; Gallou-Kabani et al. 2007), rats (Ramirez and

Friedman 1990; Shafat et al. 2009) and humans (Lissner et al. 1987). Therefore, it

seems that subjects fed a high fat diet are unable to regulate their food intake to

meet their needs and develop obesity as a consequence.

Interestingly, the hyperphagia associated with high fat diet does not seem to be due

to fat itself but rather to the energy density of the diets. Fat is characterised by a high

energy density (in kcal per g of macronutrient: fat, 9; carbohydrate, 4; protein, 4) and

thus, high fat diets are often high in energy density. Ramirez and Friedman fed rats

either a low fat or a high fat diet but presenting the same energy density. Rats fed the

high fat diet then presented decreased body weight and energy intake compared to

the mice fed the low fat diet (Ramirez and Friedman 1990). This result has been

confirmed by others in rats (Paulino et al. 2008). Therefore, as underlined by

Warwick and Schiffman, who reviewed 40 studies comparing the effects of high fat to

high carbohydrate diets, when the caloric density of the diets was similar (density of

the high fat diet less than 25 % greater than high carbohydrate diet), only 5 out of 10

studies observed greater weight gain in high fat fed animals whereas when the high

fat diet had an energy density at least 25 % greater than the high carbohydrate diet,

then 28 out of 30 studies observed a greater weight gain in the high fat fed animals

(Warwick and Schiffman 1992). These findings have been confirmed in humans as

(32)

associated with high fat diet feeding is abolished when the diets provided are

matched with respect to caloric density. To summarize, the hyperphagia associated

with high fat diet seems to be due to the high energy densities of high fat diets and

not because of the fat content of the diet per se. Of note, contradictory results have

been reported in rats when using liquid diets (Warwick 2003).

I.I.2.3. Diet-induced thermogenesis

Diet-induced thermogenesis (DIT) has been shown to have a significant effect on the

regulation of energy balance (Himms-Hagen 1985) and mainly takes place in the

brown adipose tissue in rodents (Rothwell and Stock 1979; Cannon and Nedergaard

2004). Mercer and Trayhurn showed that mice fed a high fat diet rich in corn oil,

meaning a high content of PUFA, exhibited increased energy expenditure, as

revealed by an enhanced total thermogenic activity of the BAT, compared to mice fed

a low fat diet (Mercer and Trayhurn 1987). Interestingly, the mice fed a high fat diet

rich in beef tallow, meaning a high content of SFA, did not show any evidence for an

increased DIT which could partly explain why they displayed greater body weight

compared to the 2 other groups. Differences in DIT could also partly account for the

differences in energy assimilation efficiencies since mice fed the corn oil diet retained

only 18 % of the excess energy intake in the carcass whereas mice fed the beef

tallow diet retained 77 %, despite similar total energy intakes. A decrease in DIT

induced by a diet rich in SFA as compared to diets rich in MUFA or PUFA has been

confirmed in rats as well (Takeuchi et al. 1995). Altogether, these results point at

differences in DIT as a function of the fat amount, or possibly the total energy intake

since DIT has been associated with overfeeding, and the fatty acid composition of

the diet which influences the obesity state as well (Corbett et al. 1986). BAT has long

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activity, and its role in the energy balance has been neglected until recently where

positron emission tomography demonstrated that adult humans had significant

depots of metabolically active BAT (Cypess et al. 2009; Saito et al. 2009; van Marken

Lichtenbelt et al. 2009; Virtanen et al. 2009). Therefore, there is currently a renewal

of interest for the role of the BAT in obesity in humans.

The term high fat diet actually encompasses a fairly wide spectrum of diets. In a 1992

review, Warwick analysed 40 studies comparing the effects of high fat and high

carbohydrate diets (Warwick and Schiffman 1992) and reported differences in fat

content of the diets used ranging from 28 to 84 % of total energy. The diets were also

characterised by different fat sources (corn oil, lard, tallow and others, hence

providing quite different fatty acid profiles). Therefore, an analysis of feeding trials

with animals on high fat diets has to take into account not only the energy density

and the fat content but also the role of the fatty acids provided on the development of

obesity.

I.I.2.4. The effect of the dietary fat intake on the development of obesity

The fat content of the diet has been shown to be a major determinant of body weight

in mice fed ad libitum (West et al. 1995). In this study, the authors fed two strains of

mice, AKR/J and SWR/J, with increasing levels of fat (15, 30, 45 kcal %) and

observed that dietary fat content was strongly associated with body weight gain and

a very marked increase in the weight of adipose tissue depots. However, these

effects did not become obvious in the SWR/J mice, addressing the importance of

genetic predisposition on the development of obesity. Boozer et al. fed rats

increasing amounts of dietary fat (12, 24, 36, and 48 energy %) in quantities matched

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body weight (although the diet by time interaction was significantly different) but the

absolute weights of the white adipose tissue correlated with the amount of dietary fat.

The authors concluded that dietary fat promoted adiposity, independently of the

energy intake. Pair-feeding studies are the gold-standard to determine the relative

contributions of hyperphagia versus metabolic effects of dietary fat in inducing

obesity. Although some studies reported that animals fed an isocaloric high fat diet

had greater body weight gain than animals fed a control or low fat diet (Wade 1982;

Oscai et al. 1987), other data led to the conclusion that there was no difference

(Woods et al. 2003).

I.I.2.5. Ketogenic diets

If there is a positive relationship between increasing levels of dietary fat and the

development of obesity, it actually only holds true within a defined range of fat

content as shown by experiments using ketogenic diets. Ketogenic diets typically

consist of at least 80 % of calories from fat with minimal requirements for protein and

marginal levels of carbohydrates. In mice fed ad libitum, a ketogenic diet (95 kcal %)

has been shown to promote weight loss compared to mice fed a control or a high fat

diet (45 kcal %) although they ingested similar levels of energy. This effect was due

to increased energy expenditure. Interestingly, the ketogenic diet not only prevented

mice to develop obesity but was also able to reverse obesity in mice previously fed a

high fat diet (Kennedy et al. 2007). Such ketogenic diets have proven to be more

efficient for weight loss compared to low fat diets in obese humans, even if only 68 %

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I.I.2.6. The effect of dietary fat composition on the development of obesity Diets high in fat not only differ with respect to total fat content but also in their fat

sources and thereby their fatty acid profiles. For example, beef tallow, butterfat or

pork lard are rich in SFAs, coconut oil is rich in medium-chain SFAs, olive oil is rich in

MUFAs, corn oil is rich in omega-6 PUFAs and fish oil is rich in omega-3 PUFAs.

Therefore, it is difficult to determine the effects of fat quality (SFA, MUFA, PUFA) on

the development of obesity, as the fat sources used always consist of a mixture of

different fatty acids. Moussavi et al. reviewed the possible association between types

of fatty acids in diets and weight change (Moussavi et al. 2008). In animal studies,

diets rich in SFA (beef tallow and lard) seem to initiate a greater weight gain, as

reported for mice (Buettner et al. 2006) and for rats (Mercer and Trayhurn 1987;

Takeuchi et al. 1995). Fish oil, rich in PUFA, notably the omega-3 fatty acids

eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have mainly been

shown to promote weight loss, except in one study (Awad et al. 1990). Only a few

epidemiological studies have been carried out in humans and the results are still

contradictory with regard to the quality of the dietary fatty acid patterns provided in

the diet and the effects on obesity development (Moussavi et al. 2008).

I.I.2.7. Strain differences

As already addressed, the association of dietary fat intake and obesity in mice is

strain-dependent (Fenton and Carr 1951). West et al. showed that the murine AKR/J

strain is obesity-prone whereas the murine SWR/J strain is obesity-resistant,

although when enough fat was provided in the diet, their body fat eventually

increased (West et al. 1995). Interestingly, in a following study, the AKR/J mice were

shown to have preference for fat while the SWR/J mice preferred other diets

(36)

of high fat diets on the development of obesity has been observed in rats as well

(Schemmel et al. 1970; Svenson et al. 2007). Svenson et al., from the Jackson

Laboratory, carried out a comprehensive study in 2007 where they fed 43 inbred

mouse strains for 8 weeks with a high fat diet, revealing large differences with

regards to body weight or fat mass gain as shown in Figure 1.1.

Figure 1.1. Body fat mass as a function of body weight in 43 strains of inbred mouse fed a high fat diet for 8 weeks (Svenson et al. 2007).

I.I.3. Cafeteria diet in diet-induced obesity

I.I.3.1. Is a high fat diet the most appropriate model to simulate a Western diet in human obesity?

High fat diet feeding in rodents induces changes in body weight and food intake

(37)

ongoing over whether dietary fat per se determines body fat accumulation or not.

Willett has pointed out, based on large epidemiological studies, that obesity has

increased in the last decades in the US although fat intake has decreased. Moreover,

some studies did not reveal any relation between dietary fat intake and body fatness ,

and furthermore, a reduction in fat intake had little effect, if any, on the reduction in

body weight (Willett 1998b, 1998a). To date, this debate is still open (Bray and

Popkin 1998; Willett 2002). However, a Western diet not only provides a higher

dietary fat intake but may be best characterized by a hyperphagia, providing also

high intakes of carbohydrates and proteins.

I.I.3.2. The cafeteria diet: a high fat / high sugar diet inducing hyperphagia It appears more sensible to induce obesity not only by increasing the amount of

dietary fat but also by inducing hyperphagia. Cafeteria diets have been introduced in

this respect: animals are offered a choice of several palatable food items of varied

composition, appearance and texture in addition to their nonpurified diet (Sclafani

and Springer 1976). These diets have been shown to induce obesity in a very

efficient manner, driven by hyperphagia, in rats and mice (Sclafani and Springer

1976; Rothwell and Stock 1988). For example, Rothwell and Stock reported an 80 %

increase in energy intake in rats fed a cafeteria diet compared to animals on control

diets, although the weight gain was only 27 % greater than that of the control

animals. (Rothwell and Stock 1979) This paradox was explained by an increase in

DIT in the cafeteria-fed animals, which could thus partly compensate for the excess

(38)

I.I.3.3. The effect of flavour on hyperphagia

Since cafeteria diet items are characterised by variations in flavour, texture or

macronutrient composition, it is difficult to determine what factor(s) induce

hyperphagia. The question of whether flavour variations induce hyperphagia remains

controversial. Treit et al. fed rats 2 hours per day with powdered chow flavoured with

one of 4 options, either changed every 30 min (variety day) or not (control day). The

rats given different flavours showed an approximate increase of 25 % in food intake

(Treit et al. 1983). Rolls et al. also showed that this effect was more pronounced

when rats were given the palatable items simultaneously than in succession (Rolls et

al. 1983). Nevertheless, Naim et al. did not report any effect of flavour variety on

body weight gain or energy intake when they fed rats for 23 days three control diets

with different added flavours as compared to a control diet (Naim et al. 1985). Thus, it

is not yet clear if flavour variety has an effect on energy intake or body weight gain.

I.I.3.4. The limits of the cafeteria diet

In a 1987 article in the Journal of Nutrition, Moore critically assessed the use of

cafeteria diets for studies on thermogenesis (Moore 1987). As cafeteria foods are low

in vitamins and minerals and the animals tended not to consume enough of the

nutritionally adequate nonpurified diet, the animals could face deficiencies. Moreover,

the animals usually did not eat the same items and therefore the composition of the

diet could greatly differ from one animal to another, which could affect the outcomes

of the study since the diet factor was not fully controlled. In a subsequent issue of the

Journal of Nutrition, Rothwell and Stock, the most prolific users of the cafeteria diet, convincingly dismissed Moore’s criticisms, reporting a 20 % energy intake from the

nonpurified diet and similar coefficients of variation for the different macronutrients in both control and cafeteria fed animals. Nonetheless, they acknowledged that “the

(39)

major drawbacks of the cafeteria diet are the variations in nutrient composition and the poor control over this factor” (Rothwell and Stock 1988). Controlling this factor is

actually possible, albeit painstaking and tedious (Shafat et al. 2009).

I.II. Physiological effects of high fat diets

I.II.1. The adipose tissue

Feeding a high fat diet induces a weight gain and most of this excess weight is based

on accumulated fat. However, this fat accumulation has a variety of physiological

effects, since not only does the adipose tissue expand but this organ also secretes a

large number of endocrine and paracrine factors.

I.II.1.1. Remodelling of the adipose tissue

High fat feeding elicits an increase in adipocyte size (hypertrophy) and number

(hyperplasia) (Faust et al. 1978; Berke and Kaplan 1983; Corbett et al. 1986). This

hyperplasia is not affected by food restriction, contrary to the adipocyte size, which

might indicate permanent deleterious effects of high fat diets on body weight (Rolls et

al. 1980). The consequences of this remodelling are also linked to the capacity of the

adipose tissue for the secretion of adipokines and cytokines (Huber et al. 2006).

I.II.1.2. Insulin resistance

Lavau et al. fed rats for one week either a low fat or a lard-based high fat diet. They

reported significantly decreased rates of glucose transport into the adipocytes of rats

fed the high fat diet compared to rats fed the low fat diet but the effect of insulin on

(40)

glucose uptake in high fat fed rats, but far less marked and only restricted to the

epididymal fat pad when stimulated by insulin, has been reported by Storlien et al.

(Storlien et al. 1986). Maegawa et al. also presented data on a decreased glucose

uptake but only upon insulin stimulation in high fat fed rats (Maegawa et al. 1986).

Wilkes et al. did not observe any effect of a high fat diet with a balanced fatty acid

profile but a decrease of the insulin-stimulated glucose uptake at high insulin

concentrations was seen with a high fat diet rich in PUFA (Wilkes et al. 1998). The

adipose tissue is generally considered to develop a mild insulin resistance upon high

fat feeding but very little is actually known on the impairments in the underlying

insulin signalling mechanism in the adipocyte (Anai et al. 1999; Park et al. 2005).

I.II.1.3. Lipoprotein lipase activity

Lipoprotein lipase (LPL) is the enzyme catalyzing the release of free fatty acids and

triacylglycerol from circulating triacylglyceride-rich lipoproteins to adipose tissue and

muscle. LPL activity has been shown to be enhanced in high fat fed mice in inguinal

and mesenteric fat pads but lipase activity did not show any association with insulin

levels (Surwit et al. 1995). Rossmeisl et al. observed an increase in LPL activity in

the epididymal fat depots of mice fed a high fat diet for 12 weeks. LPL activity

showed a 2-fold increase per unit of weight of tissue and a 4-fold increase if the

entire depot was considered (Rossmeisl et al. 2005). This increase in lipoprotein

lipase activity could promote the storage of excess lipids in adipose tissue.

I.II.1.4. De novo lipogenesis

Lavau et al. also determined the fate of glucose and observed that glucose

incorporation into CO² and fatty acids was decreased in rats fed a high fat diet.

(41)

dramatically increased, including for glucose incorporation into lipids, and the authors therefore concluded that “high fat feeding markedly decreases the adipocyte’s

responsiveness to insulin”. They subsequently measured lipogenic enzyme activities

which were all massively reduced. Hence, lipogenesis is already reduced in mice fed

a high fat diet for one week (Lavau et al. 1979). This effect on adipocyte lipogenesis

and lipogenic enzymes was confirmed in rats fed during 3 or 7 weeks from weaning

onwards with a high fat diet (Berke and Kaplan 1983) and was also partly confirmed

on transcription level in mice based on microarray analysis (Moraes et al. 2003).

I.II.1.5. Adipokines

The main function of adipose tissue has long been thought to be solely its energy

storage capacity. However, during the past years, considerable advances have been

made in defining its functions as an endocrine organ (Zhang et al. 1994; Kershaw

and Flier 2004) Leptin was the first adipokine discovered (Zhang et al. 1994) and was

shown to inhibit food intake and stimulate energy expenditure (Havel 2000). In mice

fed a high fat diet, leptin levels were elevated and positively correlated with body

weight (Ahrén 1999; Bullen et al. 2007). Adiponectin is the only adipokine currently

known to be negatively correlated with body mass and its decrease has been

associated with the progression of metabolic syndrome (Hauner 2005). The effects of

high fat diet feeding on adiponectin secretion are still controversial since adiponectin

plasma levels have been reported to be either unaffected upon high fat diet feeding

or elevated (Barnea et al. 2006; Bullen et al. 2007; Lee et al. 2009). Resistin, at least

in rodents, has been shown to counteract insulin activity (Steppan et al. 2001) and its

secretion has been shown to be increased in mice fed a high fat diet (Steppan et al.

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I.II.1.6. Inflammation

Analysis of gene expression levels using microarray in adipose tissue of mice fed a

high fat diet identified numerous genes significantly up-regulated that belong to

inflammatory pathways (Moraes et al. 2003). Indeed, it was shown with

immunohistochemical methods that mice fed a high fat diet showed an increased

infiltration of macrophages into their adipose tissues, forming aggregates named

Crown-like structures (CLS), and their number was significantly correlated to the

adipocyte size (Weisberg et al. 2003; Xu et al. 2003). This infiltration has been

associated with increased levels of secretion of the monocyte chemotactic protein-1

(MCP-1), a chemoattractant specific for monocytes and macrophages (Takahashi et

al. 2003; Chen et al. 2005). Interestingly, this macrophage infiltration seems to be

prevented by a diet containing fish oil, rich in omega-3 PUFA (Todoric et al. 2006).

Figure 1.2. Summary of the effects of high fat diets on the adipose tissue.

I.II.2. Liver

I.II.2.1. Hepatic steatosis

It is well established that diet-induced obesity in animals is associated with the

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disease), characterized by large vacuoles of triacylglycerides accumulating in

hepatocytes (Clarke et al. 1977; Yaqoob et al. 1995). In humans, an increase in

intrahepatic triacylglycerides (IHTG) has been associated with hepatic and peripheral

insulin resistance (Hwang et al. 2007; Korenblat et al. 2008) and is considered a

major determinant of the metabolic syndrome (Marchesini et al. 2003). Recently,

Fabbrini et al. demonstrated that IHTG content, but not visceral adipose tissue size,

was a marker of obesity-related metabolic alterations in humans (Fabbrini et al.

2009). To date, the mechanisms underlying ectopic fat distribution are not known.

I.II.2.2. Hepatic insulin resistance

The association of NAFLD and hepatic insulin resistance has been shown in

diet-induced obese animals. Using the hyperinsulinemic-euglycemic clamp technique, it

was demonstrated that the insulin-stimulated suppression of hepatic glucose

production was drastically impaired in rats fed a high fat diet (Storlien et al. 1986;

Anai et al. 1999; Li et al. 2006). The mechanisms for insulin resistance in the liver

upon high fat feeding seem to be different from those encountered in the muscle and

adipose tissue since neither the insulin receptor substrate 1 and 2 (IRS-1 and -2)

protein levels, nor their phosphorylation status, are altered. However,

phosphoinositide-3-kinase activity, acting downstream of IRS-1 and -2 to translate

insulin receptor activation into metabolic responses, is increased (Anai et al. 1999).

I.II.2.3. Hepatic de novo lipogenesis

As shown by Lavau et al. in adipocytes, glucose incorporation into fatty acids was

also decreased in the liver of rats given a high fat diet when compared to rats given a

control diet. This difference was even more marked when the incorporation was

(44)

hepatic lipogenesis (Storlien et al. 1986). Clarke et al. showed that this reduction of

de novo hepatic lipogenesis was more pronounced in rats receiving a PUFA-rich rather than a SFA-rich diet (Clarke et al. 1977).

Figure 1.3. Summary of the effects of high fat diets on the liver.

I.II.3. Intestine

The role of the intestine in the genesis of obesity has been quite underestimated,

although it is responsible for fat absorption into the circulation. Very little is known on

the effects of feeding high fat diets on the intestine and it only recently received

increased interest, notably through the use of microarrays, which allow access to the

transcriptome.

I.II.3.1. Fat absorption

In an early study, Singh et al., showed that when feeding rats for 4 weeks a

lard-based high fat diet, fecal excretion of radiolabelled lipids was significantly decreased.

Moreover, radioactive uptake of oleic acid in everted gut sacs from both jejunum and

ileum and its rate of reesterification were found to be greater in the rats fed the high

fat diet. This was confirmed by an enhanced activity for the jejunal monoglyceride

acyltransferase, a reesterifying enzyme which allows the synthesis of diglycerides in

the enterocytes from monoglycerides, a necessary step in triacylglyceride synthesis

for their export in chylomicrons (CM). Altogether, these results pointed to an

(45)

1972). This was confirmed in mice where it was shown that the fecal lipid content

was not affected by a 6-week-long high fat feeding. Thus, whether or not through an

adaptation in its absorptive capacity, the small intestine shows a very high efficiency

for fat absorption (Petit et al. 2007).

Fat absorption is a function of the total absorptive surface area times the absorptive

capacity of each enterocyte. In the work by Petit et al., microarray analysis revealed

an upregulation in genes involved in fatty acid uptake (such as the transporters

FATP-4 and CD36), intracellular fatty acid processing (the fatty acid binding proteins

I-FABP and L-FABP for example) and lipoprotein secretion (ApoA-IV and MTP for

example). This demonstrated an adaptive upregulation of genes / proteins in the

machinery to allow an enhanced lipid absorption and processing (Petit et al. 2007).

The intestinal absorptive surface area has also been found to be increased in high fat

fed mice. Petit et al. and de Wit et al. both observed an increase in the proliferation

rate of the enterocytes which could lead to an increase in villus size and ultimately in

the absorptive area (Petit et al. 2007; de Wit et al. 2008). Interestingly, de Wit et al.

also showed a downregulation of genes involved in apoptosis and an upregulation of

genes involved in cell cycle, especially in the mid and distal parts of the small

intestine. They also confirmed the increase in villus length and the total number of

cells per villus in the distal small intestine. These changes could constitute a

mechanism to support the small intestinal capacity in absorbing the bulk of dietary

(46)

I.II.3.2. Lipoprotein secretion

It has been shown that after long term feeding of a high fat diet containing long chain

fatty acids, postprandial intestinal lipoprotein secretion is increased (Cartwright and

Higgins 1999). Interestingly, this effect of diet-induced changes in postprandial

lipoprotein secretion was observed as early as after 7 days of feeding and was

characterised in the small intestine with a secretion of a smaller number of CM but of

larger size (Hernandez Vallejo et al. 2009). As the lipoprotein lipase shows a higher activity towards larger-sized particles, this could be interpreted as a way to “manage

the lipid overloading”. An adaptation of CM assembly in the intestine and secretion

was confirmed also on the transcriptome basis by increased levels of apolipoprotein

B (apoB) or the microsomal triacylglyceride transfer protein (MTP). Surprisingly, mice

fed a high fat diet for 6 weeks showed decreased blood triacylglyceride levels which

were shown to be due to an enhanced clearance from the blood, possibly through an

elevated apoCII / apoCIII ratio. It underlines the prominent role of the small intestine

in postprandial triglyceridemia (Petit et al. 2007).

I.II.3.3. Lipid oxidation

As addressed above, the small intestine reacts to a high fat diet by trying to export

more efficiently excess lipids, which can be toxic for the enterocyte (Unger and Orci

2002). De Wit et al., analysed the small intestinal transcriptome of mice fed either a

low fat or a high fat diet. They reported a significant number of biological processes

linked to lipid metabolism, cell cycle and inflammation / immune response being

significantly affected by the dietary treatment. Interestingly, they found several genes

associated with fatty acid oxidation to be upregulated in the mice fed the high fat diet,

speculating that this might act as a detoxifying process, to prevent free fatty acids

(47)

observed an upregulation of genes associated with fatty acid oxidation and

interestingly, this effect was more prominent in obesity-resistant A/J mice than in

obesity-prone C57Bl/6 mice, therefore suggesting a role of the small intestine in the

development of obesity (Kondo et al. 2006). Nonetheless, according to Gniuli et al.,

this increase in expression of genes linked to fatty acid oxidation and the increase in

enterocyte mitotic rate is not sufficient to counteract the lipotoxic effect of the high fat

diet since such a diet has been shown to induce apoptosis in rat enterocytes, the

longer the diet was being fed, the more prone the enterocytes were to lipid-induced

apoptosis (Gniuli et al. 2008). Interestingly, PUFA of marine origin (EPA and DHA)

have been found to increase lipid oxidation rates in the small intestine and might

therefore protect the small intestine from lipo-apoptosis and reduce the amount of

lipids for export via CM to the other organs (van Schothorst et al. 2009).

I.II.3.4. Hormone secretion

When de Wit et al. analysed the intestinal transcriptome of mice fed a high fat diet

compared to mice fed a low fat diet, they also found several transcripts of secreted

proteins being affected by the high fat feeding, suggesting alterations in the

communication of the gut with other organs, such as liver, muscle and adipose

tissue, via hormones and thereby underpinning the role of the small intestine in

metabolic perturbations (de Wit et al. 2008). The gut is not only an absorptive organ

but also secretes various hormones involved in energy homeostasis and satiety

(Chaudhri et al. 2008). For example, feeding a high fat diet for 30 days has been

shown to cause an elevated gastric inhibitory polypeptide (GIP) secretion (an incretin

that amplifies insulin secretion) but this response was blunted after 90 days of

feeding and was associated with a decreased insulin secretion (Gniuli et al. 2008).

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glucagon-like peptide-1 (GLP-1) (another incretin) levels and showed a reduced GLP-1

response following an oral glucose load (Anini and Brubaker 2003). It seems

therefore that feeding a high fat diet blunts the response of the small intestine with

respect to secreted peptide hormones that are involved in satiety control and in

energy homeostasis.

Figure 1.4. Summary of the effects of high fat diets on the small intestine.

I.III. The gut microbiota and obesity

The human gut hosts as many as 100 trillion microbes – collectively called microbiota

- mainly located in the colon where densities approach 1011 – 1012 cells/ml. This makes the human gut one of the most densely populated microbial habitats on Earth.

There might be thousands of species, dominated by anaerobic microorganisms,

which contain an estimated 100 times more genes than the human genome (Ley et

al. 2006a). The microbiota is able to perform functions that humans cannot

accomplish by converting undigested food components and endogenous substrates,

such as plant polysaccharides, phenolic compounds, mucin, cholesterol, biliary acids

(49)

and releases large quantities of short chain fatty acids, considered to be beneficial for

gut health. However, the gut microbiota also produces metabolites that can be

harmful. For instance, diets characterized by high saturated fat or low fibre content

have been associated with changes in the bacterial metabolism of steroids and bile

acids. Resulting metabolites, such as secondary bile acids, have been linked to

pathologies, e.g. colon cancer and cholesterol gallstone disease (Blaut and Clavel

2007). The next section will briefly review recent studies linking the gut microbiota to

obesity. This field has received great interest in the very recent years, notably through the work of Jeffrey Gordon’s group.

I.III.1. Role of the gut microbiota in fat storage

The first study that established a link between the gut microbiota and obesity was

published in 2004 by the group of Jeffrey Gordon. In this comprehensive work,

Bäckhed et al. compared parameters of energy balance in germ-free (GF) (i.e. raised

in the absence of microorganisms), conventionally raised and conventionalized

C57Bl/6 mice fed a standard rodent chow. Conventionalized mice were obtained by

spreading resuspended cecal contents of conventionally raised mice on the fur of GF

animals. 2 weeks after the colonization of the GF mice, conventionalized and

conventionally raised animals showed a 42 % increase in total body fat compared to

the GF animals. Surprisingly, the GF mice presented a 40 % higher food intake and a

27 % lower metabolic rate (as measured by O² consumption) compared to the mice

bearing a gut microbiota. As muscle and liver high-energy phosphate stores were not

affected by the microbiota, the authors concluded that the presence of

microorganisms induced futile cycles, therefore allowing a dissipation of energy

(Bäckhed et al. 2004). Moreover, GF mice appeared to be protected against

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gain in GF animals when compared to GF mice fed a low fat diet. Conventionalized

mice did not present any protection against diet-induced obesity (Bäckhed et al.

2007). Whether fed a low fat or a high fat diet, GF mice displayed an increased

locomotor activity compared to conventionalized mice, seemingly not due to the

difference in adiposity, which could contribute to the energy balance of these animals

(Bäckhed et al. 2007). Some of the proposed mechanisms which could be underlying

these effects on energy balance will be discussed next.

I.III.2. Gut microbiota and short-chain fatty acids absorption

Polysaccharide fermentation by the gut microbiota produces SCFA, e.g. acetate,

propionate or butyrate. These SCFA appear to serve as ligands for the G

protein-coupled receptor 41 and 43 (Gpr41 and Gpr43 respectively) which are located in

enteroendocrine cells of the gut. To study the contribution of Gpr41 to energy balance, Jeffrey Gordon’s group compared wild-type and Gpr41 knock-out mice

cocolonized or not with 2 organisms promoting SCFA production from dietary

polysaccharides. Knocking out Gpr41 abolished the effect of gut microbiota on fat

storage (as described in the previous section I.III.1.): GF mice, whether expressing

Gpr41 or not, had the same body weights, fat pad weights and adiposity as

cocolonized mice not expressing Gpr41. Cocolonized mice expressing Gpr41

presented significantly higher values for these phenotypic changes. Moreover,

deletion of Gpr41 reduced leptin secretion more than expected if only the associated

decrease in adiposity was considered, therefore suggesting a distinct role of Gpr41 in

microbiota-mediated leptin production. Moreover, an increased fecal energy output,

an increased uptake of monosaccharide from the gut and an increased intestinal

transit time were also observed. The latter was discussed as a consequence of a

Effets de régimes hyperlipidique et cafeteria sur le développement de l'obésité et ses désordres associés chez la souris

HAL Id: pastel-00601961

https://pastel.archives-ouvertes.fr/pastel-00601961

Effets de régimes hyperlipidique et cafeteria sur le

développement de l’obésité et ses désordres associés chez

la souris

Charles Desmarchelier

To cite this version:

Charles DESMARCHELIER

Effects of high fat and cafeteria diets on obesity development and

associated metabolic disturbances in mice

Doctorat ParisTech

T H È S E

L’Institut des Sciences et Industries

du Vivant et de l’Environnement

(AgroParisTech)

Acknowledgments

Abstract

Résumé

Keywords

Mots clés

Declaration of contributors

Table of contents

List of figures

List of tables

Abbreviations

Publications and scientific communications

General introduction

I.I. Diet-induced obesity in animal models

I.II. Physiological effects of high fat diets

I.III. The gut microbiota and obesity